As the demand for faster and higher resolution image sensors continues to increase, image sensors having multiple output channels are becoming more prevalent. This is due to the increased frame rate that can be achieved when two or more output channels are used to read out signals from the pixels. Differences or mismatches in the performance of each individual output channel, however, can introduce non-uniform effects in the operation of the image sensor. These differences or mismatches can be created during the fabrication of the output channels, or the differences can be caused by the circuitry that processes the signals read out of the image sensor.
U.S. Pat. No. 6,072,603 addresses the mismatches in output performance by using known test targets to calibrate the mismatches between multiple outputs. The calibration is typically conducted during camera manufacturing. When the calibration is complete, all of the gain and offset settings for the camera electronics, including the image sensor, are determined and saved in a non-volatile memory in the camera. Unfortunately, the accuracy of the calibration can be less than desirable because the image sensor is sensitive to the surrounding environmental or operating conditions, such as the temperature, power supply biases, or clock timing.
U.S. Pat. No. 7,423,668 calibrates an image sensor having multiple outputs through the use of an electrical-injection structure that includes a row of pixels and a reversed biased injection diode. The signal capacity of one phase in the injection structure is varied by varying the size of one barrier region within the pixels in the injection structure. Pixel signals are injected into that one phase of the injection structure by clocking the injection diode. The injected signals are used to calibrate the output channels.
Limitations to the injection structure include the need for an additional DC power supply that is turned on and off by camera electronics. The varied barrier region must be fabricated within each pixel in the injection structure, thereby adding to the cost and complexity of the fabrication process. And finally, the accuracy of the calibration can be compromised by the noise introduced by the electrical injection process itself.
U.S. Pat. No. 6,803,960 illustrates vertical optical injection structures disposed in the leading and trailing edges of a CCD image sensor. Charge is injected from the vertical optical injection structures into the single horizontal register. The injected charges are used to test the charge transfer efficiency of the CCD image sensor. These vertical optical injection structures cannot be used to calibrate multiple output channels to compensate for differences or mismatches between the output channels.